Coexistence of Type-II and Type-IV Dirac Fermions in SrAgBi

TL;DR

This paper reports the discovery of a new type of nonlinear Dirac fermion, called type-IV, in SrAgBi, which coexists with type-II Dirac fermions, expanding the understanding of fermionic excitations in condensed matter systems.

Contribution

The paper introduces the concept of type-IV Dirac fermions with nonlinear dispersion and demonstrates their coexistence with type-II Dirac fermions in SrAgBi, supported by effective models and topological surface state calculations.

Findings

Discovery of type-IV Dirac fermions with nonlinear dispersion.

Coexistence of type-II and type-IV Dirac fermions in SrAgBi.

Topological surface states associated with these Dirac points.

Abstract

Relativistic massless Weyl and Dirac fermions have isotropic and linear dispersion relations to maintain Poincar\'{e} symmetry, which is the most basic symmetry in high-energy physics. The situation in condensed matter physics is less constrained; only certain subgroups of Poincar\'{e} symmetry -- the 230 space groups that exist in 3D lattices -- need be respected. Then, the free fermionic excitations that have no high energy analogues could exist in solid state systems. Here, We discovered a type of nonlinear Dirac fermion without high-energy analogue in SrAgBi and named it type-IV Dirac fermion. The type-IV Dirac fermion has a nonlinear dispersion relationship and is similar to the type-II Dirac fermion, which has electron pocket and hole pocket. The effective model for the type-IV Dirac fermion is also found. It is worth pointing out that there is a type-II Dirac fermion near this new Dirac fermion. So we used two models to describe the coexistence of these two Dirac fermions. Topological surface states of these two Dirac points are also calculated. We envision that our findings will stimulate researchers to study novel physics of type-IV Dirac fermions, as well as the interplay of type-II and type-IV Dirac fermions.